In the manufacture of polymeric foams such as, for example, polyurethanes, polyureas, polystyrene, polyethylene and the like a heat activated blowing agent is employed to provide the desired cell structure. While it has been customary to employ a chlorinated fluorocarbon (CFC) for this purpose, the cumulative effect on the ozone layer of the atmosphere has made it desirable to utilize non-ozone depleting blowing agents. Although other gases including carbon dioxide, nitrogen, helium, ammonia, pentane, acetylene, the inert gases, air and mixtures thereof have been investigated, generally none has produced the desired cell size, structure and insulative properties obtained with the CFC's.
In the production of polyurethanes the blowing agent is mixed with a liquid monomer, or reaction component of a two part system, e.g. polyahl or isocyanate, prior to polymerization. One of the problems associated with the use of non-CFC blowing agents has been their introduction, dispersion and distribution into the liquid monomer. For a given cell structure it is known that a specific volume of blowing agent must be present however, the solubility and miscibility of the agent is an important factor with which the manufacturer must reckon.
Another problem has been the accurate determination of the volume of blowing agent actually incorporated into the liquid monomer component prior to polymerization because escape of the agent during measurement leads to erroneous determinations and, in turn, the use of incorrect amounts of the agent as corrections are made or not made.
To date the patent art provides numerous examples of apparatus and methods for using various non-CFC blowing agents in liquid monomer components. For example, the introduction of an inert gas, such as nitrogen, into a liquid reaction component of a reaction injection molding (RIM) system is taught by U.S. Pat. No. 4,157,427. In general, the gas is added to one of the precursors of a polyurethane by use of a sparger stone through which the gas is forced, under pressure. The sparger is described as a suitably sized and shaped porous rigid structure, to produce minute bubbles for better mixing, that is placed in a pipe through which the reactive component is circulated from the supply tank and then sent either to a mixing head or back to the supply tank.
U.S. Pat. No. 4,376,172 is directed toward a closed loop apparatus for controlling the addition of a gas to a liquid, such as a polyurethane precursor, in a RIM process. Additionally, means are provided for accurately measuring the amount of the gas that is added. The blowing agent or gas is added by means of a sparger which is in a stream of the reactant being recirculated from the supply tank and back to the supply tank.
Measurement of the amount of gas added to the polyurethane reactant is performed by tapping a volume of the gas-reactant mixture and holding it in a cylinder. A piston is then driven into the cylinder to check, by means of compressibility, the amount of gas which has been added.
U.S. Pat. No. 4,526,907 is directed toward a process and device for charging gas into at least one of the components combined to produce plastic foams. The reactant from one supply tank is piped through a circulation line which has a zone of compression that is higher in pressure than that in the supply tank. In this compression zone the foaming gas is added, and the mixture is subsequently forced through a throttle element to reduce the pressure before return to the supply tank. The patent also teaches that several different methods can be employed to determine the amount of gas in the gas-reactant mixture including density, partial pressure, the absorption of a beam of light, compressibility and solubility, but does not necessarily discuss means for doing so.
U.S. Pat. No. 4,906,672 is directed toward a method for the continuous manufacture of polyurethane foam. More particularly, it deals with the addition of carbon dioxide to polyurethane-forming reactants as the blowing agent and teaches that the carbon dioxide is to be dissolved into one of the reactants well before being sent to the mixing head.
Introduction is performed under high pressure, preferably 75 to 900 psig (0.57 to 6.2 MPa), in a pipe, a sufficient distance from the mixing head that uniform entrainment is achieved upon traveling from the sight of impingement to the mixing head. Once the mixture reaches the mixing head, a nozzle or series of nozzles are employed to expand the carbon dioxide-reactant mixture; however, the patent teaches that the entrainment of bubbles is to be avoided.
Finally, European Pat. No. 125,541-B discloses a device for measuring the gas charging of a liquid component used for producing synthetic plastic foam such a polyurethanes. It employs a measuring vessel, for receipt of a liquid sample periodically, and which communicates with an overflow vessel. By allowing the pressure in the measuring vessel to decrease to atmospheric, the gas laden component expands and overflows vessel which allows density to be determined.
Thus, it should be apparent that although others have employed low boiling compounds, such as, carbon dioxide, nitrogen and the like as blowing agents for polyurethane foam, apparatus and method have not been taught for the incorporation of a blowing agent in a liquid monomer, in precise amounts and bubble sizes so as to control the cell structure of the resulting foam, or for the precise measurement of the density of a mixture of blowing agent and liquid monomer, the liquefaction and solubility of the blowing agent in the mixture, or the determination of its volumetric expansion potential therefrom. Moreover, previous apparatus and methods have not been successful in providing uniform distribution of the blowing agent in the liquid monomer component, which has grossly affected the quality of the resulting foam product.